l world to-day. In such animals as
the hydra we find the first crude beginning of unorganised nerve-cells.
In the jelly-fish we find nerve-cells clustered into definite sensitive
organs. In the lower worms we have the beginning of organs of smell and
vision. They are at first merely blind, sensitive pits in the skin, as
in the embryo. The ear has a peculiar origin. Up to the fish level there
is no power of hearing. There is merely a little stone rolling in a
sensitive bed, to warn the animal of its movement from side to side. In
the higher animals this evolves into the ear.

The glands of the skin (sweat, fat, tears, etc.) appear at first as
blunt, simple ingrowths. The hair first appears in tufts, representing
the scales, from underneath which they were probably evolved. The thin
coat of hair on the human body to-day is an ancestral inheritance. This
is well shown by the direction of the hairs on the arm. As on the ape's
arm, both on the upper and lower arm, they grow toward the elbow. The
ape finds this useful in rain, using his arms like a thatched roof, and
on our arm this can only be a reminiscence of the habits of an ape
ancestor.

We have seen how the spinal cord first appears as a tube in the axis of
the back, and the cartilaginous column closes round it. All bone appears
first as membrane, then cartilage, and finally ossifies. This is the
order both in past evolution and in present embryonic development. The
brain is at first a bulbous expansion of the spinal nerve-cord. It is at
first simple, but gradually, both in the scale of nature and in the
embryo, divides into five parts. One of these parts, the cerebrum, is
mainly connected with mental life. We find it increasing in size, in
proportion to the animal's intelligence, until in man it comes to cover
the whole of the brain. When we remove it from the head of the mammal,
without killing the animal, we find all mental life suspended, and the
whole vitality used in vegetative functions.

In the evolution of the bony system we find the same correspondence of
embryology and evolution. The main column is at first a rod of
cartilage. In time the separate cubes appear which are to form the
vertebrae of the flexible column. The skull develops in the same way.
Just as the brain is a specially modified part of the nerve-rod, the
skull is only a modified part of the vertebral column. The bones that
compose it are modified vertebrae, as Goethe long ago suspected. The